The present invention relates to a device for securing bits of bone together, such as, in particular, bits of metatarsal head, for example in the context of repair using the Weil osteotomy.
The Weil osteotomy is an operation which consists in taking a line, on a metatarsal head, parallel to the sole of the foot and in shifting a bit backward, then in securing the bits of bone together.
To join small-sized bone fragments or elements together use is commonly made of surgical screws containing threaded portions intended to be screwed into the two bits of bone which are to be held together, and it is known practice for the screw threads present on such screws to be given different pitches so as to ensure firm anchorage and a compression effect that encourages the bone to take and encourages osteointegration.
These screws made, for example, of a biocompatible material, such as titanium, are difficult to handle accurately, particularly as a result of their small size. They often have a complex construction which increases their cost price. Finally, they entail the use of ancillaries, generally bulky and ill-suited to precisely checking the exact position of the screw set in place in the bone fragments.
The present invention sets out to overcome these drawbacks and to provide a device for securing bones together which is particularly easy to handle and to fit and which provides excellent consolidation.
The subject of the invention is a device for securing bits of bone together, particularly small-sized bits of bone, for example bits of metatarsal head, characterized in that it is in the form of a spindle comprising an elongate shank, followed, after a region in which the material is weakened, to allow it to be broken, by a fixator element, extending frustoconically so that it reduces in diameter in the direction of the free anterior end of the element, and, on this frustoconical surface, having a screw thread the pitch of which decreases progressively from said anterior end and of which the crests of the threads, the bottoms of which are formed by said frustoconical surface, lie inside a geometric cylinder coaxial with the axis of the spindle, said anterior end being beveled so as to form a self-drilling end, so as to allow it to be screwed self-tappingly into the bits of bone.
This results in a growing increase in the thread depth from the region of weakness toward the free anterior end.
The self-drilling end is preferably achieved by a point of the trocar type, formed, for example, of three inclined panels angularly offset by 120xc2x0.
As a preference, the shank of the spindle, which is connected to the fixator element proper by a region of weakness determining a breakable region, has a smooth or otherwise treated cylindrical surface to make it easier to hold in a rotary apparatus such as a drill.
The breakable part is advantageously produced in the form of a groove of roughly triangular cross section, the point of which determines a fairly acutely angled groove bottom, which facilitates breakage, by angular tilting, of the fixator element with respect to the shank, while at the same time being perfectly able to withstand the stresses generated as the fixator element is driven into the bits of bone.
In one preferred embodiment, the posterior part of the fixator element, which extends toward the region of weakness, has longitudinal grooves partially interrupting the threads in that region, so as to allow the introduction of corresponding fingers of a tool that allows, if need be the fixator element to be unscrewed by turning it in the opposite direction to the direction in which it is turned in order to drive it into the bits of bone.
The spindle according to the invention may thus be used to join and press together two fragments of bone after these have been respectively positioned by the surgeon, followed possibly by the drilling of a pre-hole with a spindle of smaller diameter and the introduction and compressive screwing-in of the spindle according to the invention into its final position, followed by breakage of the breakable region, and removal of the actual shank of the spindle.
In one advantageous embodiment it is possible, in order to drill the pre-hole, to use a threaded spindle of a smaller diameter, after which the spindle according to the invention is placed, this giving rise to the displacement of small volumes of bony substance between the threads, making osteointegration easier.
As a preference and according to the invention, a number of spindles according to the invention, having different dimensions in terms of the length and/or in terms of the thread crest diameter and/or in terms of the cone angle and/or in terms of the pitch are grouped together into a set.
As a preference, the spindles of one set differ in the length of the implantable fixator element, the thread crest diameter remaining unchanged.
The number of different sizes of spindle in a set or kit such as this is advantageously between 10 and 16, it being possible for the set to have one or more spindles in each size.
As a preference, the dimensions of the device according to the invention lie in the following ranges:
length of the fixator element between the tip and the bottom of the breakable groove: 9 mm to 40 mm.
cone angle of the fixator element: 0.75xc2x0xc2x10.5xc2x0.
mean diameter of the frustoconical surface: 1.2 mm.
diameter of the geometric cylinder at the crest of the thread: 2 mm to 2.5 mm.
variation in the thread pitch (from) 0.5 mm at the tip to 1.8 mm at the posterior end, it being possible for these values to vary by plus or minus 0.5 mm.
As a preference, the sections of the threads are roughly triangular with one face inclined toward the front of the fixator element and one face roughly perpendicular to the axis of the spindle toward the rear of the fixator element.